The present invention relates to the field of optical metrology. More specifically it provides a means by which repeatable reflectance measurements may be performed over a broad range of wavelengths that includes the vacuum ultraviolet (VUV) (generally wavelengths less than 190 nm) and at least one other spectral region.
Optical metrology techniques have long been employed in process control applications in the semiconductor manufacturing industry due to their non-contact, non-destructive and generally high-throughput nature. The vast majority of these tools operate in some portion of the spectral region spanning the deep ultraviolet and near-infrared wavelengths (DUV-NIR generally 200–1000 nm). The push towards thinner layers and the introduction of new complicated materials has challenged the sensitivity of such instrumentation. As a result, this has necessitated an effort to develop optical metrology equipment utilizing shorter wavelengths (below 200 nm), where greater sensitivity to subtle changes in material properties can be realized. One approach to performing optical measurements at shorter wavelengths is described in U.S. application Ser. No. 10/668,642, which discloses a system and method for a vacuum ultraviolet (VUV) reflectometer.
Virtually all optical metrology instruments incorporate some form of modeling algorithms to extract meaningful material information from the quantities they initially record. The performance of such algorithms depends heavily on the nature of the data sets they are to reduce. Data sets covering a wider range of wavelengths generally provide more constraint to fitting algorithms thereby rendering faster convergence and more accurate results.
The conventional approach to collecting reflectance data over a broad range of wavelengths covering at least two spectral regions is to employ a step and scan technique wherein a single element detector is used in conjunction with a rotating grating monochromator. Often if the range of wavelengths investigated is large enough, it may be necessary to manually change out gratings, detectors, optics and sources during the acquisition of a single broad-band data set. This approach is often time consuming and not well suited to manufacturing environments like those encountered in the semiconductor industry.
Interferometers are widely used in the infrared spectral region to collect data over a wide range of wavelengths; however, these instruments are not commonly employed in the VUV since optical and mechanical tolerances of the instrument scale with wavelength and are difficult to satisfy in this spectral region.
Wang, in U.S. Patent Application 2003/0071996, discloses a measurement system with separate optimized beam paths. Although this system enables efficient measurements to be performed over a number of spectral sub-bands, it provides no means of referencing the collected data. Hence, while signal throughput may be high, system repeatability may be quite poor. This is particularly relevant when operating in the VUV since such wavelengths are highly susceptible to atmospheric changes necessitating frequent referencing.
The collection of highly repeatable reflectance data in the VUV is perhaps best achieved using a system designed to minimize and/or altogether eliminate errors introduced by data altering environmental changes which may occur between conclusion of a calibration measurement and commencement of a subsequent sample measurement. An example of such a system is described in U.S. application Ser. No. 10/668,644. The applicant has identified that it would be desirable to extend this capability in order to facilitate its use in a reflectometer capable of acquiring data over a broad range of wavelengths including the VUV and at least one other spectral region.
The applicant has further identified that it would be desirable to ensure that data sets from each of the spectral regions comprising the entire broad band of wavelengths are collected from the same physical location on the sample and with the same spot size. Moreover, it would also be advantageous if such data sets were collected using the same orientation (i.e. angle of incidence and direction) relative to the sample in order to ensure that similar scattering conditions are encountered.
The applicant has also identified that it would be desirable if said system made use of a serial collection process wherein data from each of the spectral regions were collected sequentially to avoid stray light complications, which one would expect if a parallel process was employed.